In vertebrates, the olfactory receptors are located in the cilia of the olfactory sensory neurons. In insects, olfactory receptors are located on the antennae. Sperm cells also express odor receptors, which are thought to be involved in chemotaxis to find the egg cell.
Rather than binding specific ligands like most receptors, olfactory receptors display affinity for binding a range of odor molecules. Once the odorant has bound to the odor receptor, the receptor undergoes structural changes and it binds and activates the olfactory-type G protein on the inside of the olfactory receptor neuron. The G protein (Golf and/or Gs) in turn activates the lyase - adenylate cyclase - which converts ATP into cyclic AMP(cAMP). The cAMP opens cyclic nucleotide-gated ion channels which allow calcium and sodium ions to enter into the cell, depolarizing the olfactory receptor neuron and beginning an action potential which carries the information to the brain.
There are a wide range of different odor receptors, with as many as 1,000 in the mammalian genome. Olfactory receptors may make up as much as 3% of the genome. Only a portion of these potential genes form functional odor receptors. According to an analysis of the Human genome project, humans have approximately 400 functional genes coding for olfactory receptors and approximately 600 putative pseudogenes remain.
The reason for the large number of different odor receptors is to provide a system for detecting as many different odors as possible. Even so, each odor receptor does not correspond to just one odor. Each individual odor receptor is broadly tuned to be activated by a number of similar structures. Like the immune system, this system allows molecules that have never been encountered before to be characterized. Also most odors activate more than one type of odor receptor. This aspect provides for the identification of an almost limitless number of different molecules.
History and research
In 2006 it was shown that another class of odorant receptors exist for volatile amines. This class of receptors consists of the trace amine-associated receptors (TAAR) with the exception of TAAR1 which is a receptor for thyronamines.
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- Olfactory Receptor Database
- Human Olfactory Receptor Data Exploratorium (HORDE)
- Olfactory+Receptor+Protein at the US National Library of Medicine Medical Subject Headings (MeSH)